Along with the greatly increased use of computers for offices and for manufacturing facilities, there has developed a need for a cable which may be used to connect peripheral equipment to mainframe computers and to connect two or more computers into a common network. Of course, the sought-after cable desirably should provide substantially error-free transmission at relatively high rates.
A number of factors must be considered to arrive at a cable design which is readily marketable for such uses. The jacket of the sought-after cable should exhibit low friction to enhance the pulling of the cable into ducts or over supports. Also, the cable should be strong, flexible and crush-resistant, and it should be conveniently packaged and not unduly weighty. Because the cable may be used in occupied building spaces, fire-resistance also is important.
The sought-after data transmission cable should be low in cost. It must be capable of being installed economically and be efficient in terms of space required. It is not uncommon for installation costs of cables in buildings, which are used for interconnection, to outweigh the cable material costs. Building cable should have a relatively small cross-section inasmuch as small cables not only enhance installation but are easier to conceal, require less space in ducts and troughs and wiring closets and reduce the size of required, associated connector hardware.
Of importance to the design of local area network copper conductor cables are the speed and the distances over which data signals must be transmitted. In the past, this need has been one for interconnections operating at data speeds up to 20 kilobits per second and over a distance not exceeding about 15 feet. This need has been satisfied in the prior art with single jacket cable which may comprise a plurality of insulated metallic conductors that are connected directly between a computer, for example, and receiving means such as peripheral equipment. Fire-resistance, relatively modest costs and suitable mechanical properties have been achieved with such prior art metallic conductor cables.
In today's world, however, it becomes necessary to transmit data signals at much higher speeds over distances which may include several hundreds of feet. Currently, equipment is commercially available that can transmit 16 Mbps data signals for 300 or 400 feet. Even at these greatly increased distances and data rates, the desired transmission must be substantially error-free and at relatively high rates. Further advances in data rate/distance capability are becoming increasingly difficult because of crosstalk between the pairs of commercially available cables.
To satisfy present, as well as future needs, the sought-after cable should be capable of suitable high frequency data transmission. High frequency herein is intended to mean 0.5 MHz or higher. This requires a tractable loss for the distance to be covered, and crosstalk performance and immunity to electromagnetic interference (EMI) that will permit substantially error-free transmission. Also, the cable must not contaminate the environment with electromagnetic interference.
In the prior art, transmission has been carried out on cables in which conductors insulated with polyvinyl chloride (PVC) have been used. It has been found that polyvinyl chloride insulation, although having acceptable flame retardant properties, results in transmission losses which are undesirably high for the transmission of high frequency signals. This may be overcome somewhat by increasing the gauge size of the metallic conductor portion of the insulated conductor, but, as should be apparent, this is not a desirable alternative.
Also, it has been customary to insulate metallic conductors by extruding a skin of polyvinyl chloride over a foamed polyethylene insulation material. This has been referred to as a foam-skin arrangement. Pairs are made by twisting together two of the insulated conductors. Such cables including one or more twisted pairs may be enclosed by an inner jacket, a metallic shield disposed over the inner jacket and an outer jacket disposed over the shield. Typically the outer jacket has been comprised of polyvinyl chloride.
The last-described prior art cable has disadvantages associated therewith. Foamed polyethylene disposed adjacent to the metallic conductor and having a cover of a solid PVC insulation material has acceptable fire-resistant properties. However, the twisting of the conductors into pairs causes the foam insulation to be crushed, resulting in the spacing between the metallic conductors being reduced with accompanying transmission losses. This problem is exacerbated when a short twist arrangement, which is particularly likely in a local area network environment, is used. See U.S. Pat. No. 4,873,393 which issued on Oct. 10, 1989 in the names of H. W. Friesen and Wendell G. Nutt. Further, it has been found that in prior art foam-skin insulation arrangements wherein PVC has been used as a skin, there have been undesirable losses at high frequency. Also, in a shielded cable in which PVC has been used as a inner jacket and in which each conductor is insulated with an inner layer of polyethylene and an outer layer of flame-retardant polyethylene, high frequency loss has been experienced. Also, of course, it is desirable to be able to eliminate the metallic shield, the forming of which requires additional materials and lower manufacturing line speeds.
What is needed and what seemingly has not been provided by the prior art is a cable which includes an insulation and jacketing system which causes the cable to be suitable for the transmission of high frequency signals at a suitably low loss. The sought-after cable also should be one which is acceptably fire-resistant so that it may be used in buildings. Materials used in the sought-after cable should be readily available and not impose an unduly high price penalty on the resulting product. Also, the insulation system must be such that it is not crushed when two of the insulated conductors are twisted together with a relatively short twist length.